a) Field of the invention The present invention relates to a moldable refractory composition for use to prepare refractory pieces that can handle molten metals, especially molten aluminum.
More particularly, the invention relates to a refractory composition which, after casting, can be unmolded within a very short period of time that can be as small as 10 minutes.
b) Brief description of the prior art
There are numerous refractory products known in the art, which are resistant to molten metals, like molten aluminum, and are therefore useful for handling the same. Amongst these refractory products, one can mention those prepared from compositions containing one or more heat resistant inorganic materials, like asbestos fibers, in admixture with a colloidal silica that is used as a primary binder.
As an example of such refractory products and compositions for preparing the same, reference can be made to U.S. Pat. No. 3,224,927 of 1965 to BROWN et al, which discloses a composition for preparing heat resistant products that can be formed and/or molded into rods, tubes or blocks. This composition comprises inorganic fibers, preferably asbestos fibers, bonded with from 2 to 20% by weight of colloidal silica and from 0.20 to 1.5% by weight of a positively charged starch, the above percentages being based on the weight of inorganic fibers in the composition. The above mentioned, positively charged starch is used inter alia as a secondary binder to impart strength to the cast products until a silica bonding can be developed by heating.
As another example of refractory products of the above mentioned type and compositions for preparing the same, reference can also be made to U.S. Pat. Nos. 4,873,209 of 1989 and 5,033,721 of 1991, both to GNYRA, which disclose a castable composition for preparing light weight refractory products that can be used in metal foundries especially as cementing material or as surface finishing or "face-off" material to form a protective coating layer to protect an underling article. The composition disclosed in these patents comprises a mixture of ceramic fibers and wollastonite powder and an aqueous colloidal silica having a solid content of 5 to 25% by weight. It can also comprise milled and/or screened vermiculite to adjust the density and strength of the final product. In use, the composition disclosed in the above patents is efficient. However, it has the drawback of taking about 24 hours to dry and harden at room temperature, thereby making the casting process very time consuming.
It is already known in the art that one may control and actually accelerate the setting of ceramic coating compositions containing colloidal silica as a binder, by adding thereto a phosphorus compound like phosphoric acid and its salts, especially trivalent metal phosphates. In this connection, reference can be made to U.S. Pat. Nos. 2,995,453 of 1961 to NOBLE et al; 3,041,205 of 1962 to ILER and 3,650,783 of 1972 to YATES. However, the compositions disclosed in these patents, are essentially useful as refractory binders, coatings or adhesives, and not for the preparation of cast pieces.
On the other hand, U.S. Pat. No. 5,362,690 of 1994 to DELVAUX et al discloses and claims a castable refractory composition for use in the prosecution of cast refractory pieces, having a phosphate salt as a primary binder. The composition comprises 1 part by weight of a phosphoric acid solution having a concentration ranging from 40 to 85% and its primary acidic function partially neutralized by reaction with vermiculite. It also comprises from 0.1 to 2.5 parts by weight of a mineral, preferably wollastonite, that is refractory and of fibrous structure and may react with one part of the acidic functions of the phosphoric acid that are still free or not neutralized yet. It further comprises from 0.1 to 2.5 parts by weight of another mineral such as serpentine, MgO or synthetic olivine, containing at least one cation that may react with another part of the acidic functions of the phosphoric acid that are still free or not saturated yet and from 0.28 to 1.45 parts by weight of water. When the composition disclosed in this patent is processed, it passes from a granular state to a plastic state, and then to a liquid state, without need for any external heating. At that time, it may be cast to produce refractory pieces having good thermal insulation characteristics, and in particular, a very good behaviour with respect to molten aluminium.